Securely adding signatures to pdf documents

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Hi,

We have a requirement for adding scanned signatures to pdf documents we generate. There are multiple possible signatures that must be added based on the person selected during the workflow (a responsible person).
What would be a secure way of adding these "pictures" or scans of signatures to the generated pdf?
One option we tried was creating a form field, that generated a URL with the path to the image of the signature based on the person selected as a responsible person, but this solution is not considered secure, since the signatures must be stored somewhere that people can access (for example a document library in SharePoint).

Any other possible solutions?
I should add we are using classic forms on SharePoint.

Thanks.